Recently, there has been much active development of liquid crystal devices for use in flat panel liquid crystal displays, most notable of which have been the advances in monolithic driver panel design. Parallel to such developments, various types of thin film transistors (hereinafter referred to as TFTs) made of polycrystalline silicon have also been proposed for use in the monolithic driver panel.
In a liquid crystal display device which employs the above-mentioned TFTs and is driven by an active-matrix driving method, each pixel driver includes a supplemental capacitive element (condenser), namely, a signal accumulating capacitor. The structure of such liquid crystal display devices, which is also disclosed by example in Japanese Examined Patent Publication No.1-33833(1989), is further explained in the following paragraphs with reference to FIGS. 5 and 6.
A pixel of a conventional liquid crystal display device comprises a substrate 51 and a semi-conductive layer 56 placed atop of substrate 51. Semi-conductive layer 56 is formed through a predetermined process so that it provides a source area 53 and a drain area 54 as part of TFT 52, and an electrode 55 for a supplemental capacitor 60 (i.e., the supplemental capacitive element). A gate insulating film 57 of TFT 52 is formed on top of semi-conductive layer 56. Also, a gate electrode 58 of TFT 52, another electrode 59 of supplemental capacitor 60, and an inter-layer insulating film 61 are formed on gate insulating film 57. A pixel electrode 62 for pixel 65 (which also exhibits inherent electrical capacitance) is formed on top of inter-layer insulating film 61. A source electrode 63 and a drain electrode 64 are formed on top of source area 53 and drain area 54, respectively. Pixel electrode 62 is electrically connected to drain electrode 64.
According to the above structure, however, supplemental capacitor 60 includes a only single thin insulating film, namely, gate insulating film 57 alone, between two electrodes 55 and 59. Thus, making gate insulating film 57 thinner (which would enhance the performance of TFT 52) results in making the insulating film (i.e., gate insulating film 57) of supplemental capacitor 60 thinner as well. Since insulating film (gate insulating film 57) portion for supplemental capacitor 60 is larger in area than the insulating film portion for TFT 52, making the film thinner more readily causes defects such as pin holes to occur. In other words, if gate insulating film 57 is thinned down to enhance the performance of TFT 52, supplemental capacitor 60 becomes less reliable.
To eliminate this problem, gate insulting film 57 of TFT 52 may be thinned while thickening gate insulating film 57 of supplemental capacitor 60. However, this increases the number of stages in the manufacturing process approximately two-fold as a result (i.e., the number of the stages in the photolithographic process used during device fabrication, including a patterning stage and a film forming stage, is increased approximately two-fold). Consequently, this is an undesirable situation where the yield of liquid crystal display devices during manufacturing decreases and manufacturing costs increase.